Sites and sensor deployments
SeaFET pH sensors (Martz et al. 2010) and Conductivity, Temperature, Depth, and Oxygen
sensors (CTDO sensors, Sea-Bird Electronics 37-SMP-ODO MicroCAT C-T-ODO (P) Recorder)
were deployed at three sites along the northern Channel Islands: (1) Anacapa Island
Landing Cove pier (ALC, 34° 00.985’N, 119° 21.724’W) in a marine reserve with kelp forest
habitat, (2) Santa Cruz Island Prisoner’s Harbor pier (PRZ, 34° 01.225’N, 119° 41.057’W)
surrounded by a large shallow eelgrass bed (Zostera pacifica), and (3) San Miguel Island
northern subtidal mooring (SMN, 34° 03.417’N, 120° 20.731’W) at 6 m in open water over a
sandy bottom at 18 m depth (Fig. 1). Sensors at ALC and PRZ were deployed at 3 - 4 m
depth and less than 1 m from the benthos on a pier piling.
In May 2013, CTDO sensors were deployed in addition to pH sensors at ALC and PRZ. CTDO
sensors were actively pumped through an anti-fouling passage and temperature, salinity,
pressure, and dissolved oxygen were recorded every 15 min. In August 2013, the sensor
array from PRZ was moved to SMN for a one-year overlapping period of data collection
with ALC. At each site, sensors were swapped every 2 - 3 months. SeaFET sensor surfaces
did not exhibit biofouling upon recoveries.
Calibration samples for SeaFET sensors were collected 1 – 8 times during each 2 - 3
month deployment via SCUBA, free diving, or a GO-FLOW (General Oceanics) bottle drop
from the pier following Standard Operating Procedures (SOP) 1 (Dickson et al. 2007).
Multiple calibration samples were taken to quantify spatio-temporal mismatch of the
sensor data and bottle sample (Bresnahan et al. 2014). Samples were fixed immediately
with saturated mercuric chloride. Water samples were analyzed for pH25 °C (SOP 6b, using
m-cresol purple from Sigma-Aldrich®), total alkalinity (SOP 3b, using open-cell titrator
Mettler-Toledo T50) (Dickson et al. 2007), and salinity (YSI 3100 Conductivity
Instrument) when no corresponding salinity measurements was available from a CTDO
sensor. In-situ pHT (total hydrogen ion scale) was calculated using either temperature
recorded by the SeaFET or CTDO sensor when available (CO2Calc (Robbins et al. 2010); CO2
constants from (Dickson and Millero 1987; Mehrbach et al. 1973)). All pH data are
reported as pHT (“SF_pHint_tot”).
SeaFET data processing followed methods from (Bresnahan et al. 2014) for single and
multiple calibration samples using Matlab (R2012b, R2014a). When SeaFET deployments were
paired with CTDO sensors (May 2013 – September 2014), temperature data from the CTDO
sensors was used to correct for the offset associated with the uncalibrated SeaFET
thermistor. CTDO sensors underwent factory calibration at the start and end of the
project. Sensors were rinsed with DI water and dilute Triton-X, between deployments.
CTDO data was interpolated onto the SeaFET sampling period and all data are reported in
Coordinated Universal Time, unless specified otherwise. One 24-hour gap of CTDO data was
interpolated to match the deployment length of the pH sensor at ALC when necessary for
computations. Rare instances where pH declined to below pH 7.7, within two observations
and independent of changes in temperature, were removed for quality control. Oxygen
saturation recorded by the CTDO sensor was converted to dissolved oxygen (DO) μmol kg-1
using the oxygen solubility combined fit conversion equation from (García and Gordon
Post-calibration of CTDO sensors revealed negligible drifts in oxygen, salinity, and
temperature. A total of six in-situ water samples were collected for Winkler
determination for dissolved oxygen (Wetzel and Likens 1991) and showed a mean 0.9 +/- 0.9
% positive offset from sensor observations (maximum offset was 2.4 %). Post-calibration
indicated oxygen sensor drift of less than 1-2 % across the three instruments. The data
were not corrected for drift of the oxygen sensor and accuracy for this data is +/- 2 %. A
small drift in conductivity resulted in a salinity accuracy of +/- 0.02 psu, and
temperature drift was less than 0.001 degree C for all sensors with a reported accuracy of +/-
0.002 °C. Data were not corrected for sensor drift.
Errors in pHT measurement are largely due to the use of unpurified m-cresol dye (0.02,
(Liu et al. 2011)), user error (+/1 0.006, (Kapsenberg et al. 2015)), and spatio-temporal
mismatch of the calibration sample as determined from multiple calibration samples in
one deployment (+/- 0.010 for SMN, ± 0.026 for PRZ, +/- 0.005 for ALC). The resultant
estimated standard uncertainty of pH data differed by site and was +/1 0.023 (SMN), +/-
0.033 (PRZ), and +/- 0.022 (ALC).
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